Mail PTR Records

[Nathan Olson]

RFC 2821

4.1.4 Order of Commands

The SMTP client MUST, if possible, ensure that the domain parameter
to the EHLO command is a valid principal host name (not a CNAME or MX
name) for its host.  If this is not possible (e.g., when the client's
address is dynamically assigned and the client does not have an
obvious name), an address literal SHOULD be substituted for the
domain name and supplemental information provided that will assist in
identifying the client.

An SMTP server MAY verify that the domain name parameter in the EHLO
command actually corresponds to the IP address of the client.
However, the server MUST NOT refuse to accept a message for this
reason if the verification fails: the information about verification
failure is for logging and tracing only.